In the case of such intravascular or intracardial implants such as stents, electrode lines or the like, the problem which frequently arises is that uncontrolled cell growth is induced in the surrounding body tissue of the patient due to the foreign body which is not sufficiently biocompatible and which is represented by the implant, or due to injuries which occur in the implantation procedure in the adjoining body tissue.
That uncontrolled cell growth is undesirable. Thus for example in the case of intravascular implants it can have the result that the vessel passage is severely constricted by the proliferating tissue or is even completely closed in the short or long term. That is to be prevented in any case as such a vessel constriction or closure can under some circumstances bring about a life-threatening condition of the patient.
In order to counter that problem, for example, U.S. Pat. Nos. 6,153,252 and 6,206,916 B1 each disclose an implant of the general kind set forth, in the form of a stent, in which a cell proliferation-inhibiting substance is given off by a bioresorbable coating on the stent in order to suppress the uncontrolled cell growth of the vessel wall in the region of the stent. In order to delay the delivery of active substance or to cause it to take place more slowly, it is provided in that respect inter alia that a bioresorbable cover layer is disposed over the active substance-bearing coating.
The cell proliferation-inhibiting substances which are used in the known stents are generally operative in such a way that no cell growth worth mentioning occurs in the surroundings of the implant.
A further problem in connection with such intravascular or intracardial implants lies in their thrombogenic action, that is to say the increased risk of thrombosis that they entail. Depending on the respective degree of their biocompatibility, clots can comparatively easily be formed at the surface thereof. On the one hand they can also result in constriction or closure of the vessel portion in question, with the consequences which have already been referred to above. On the other hand such clots can also become detached from the location at which they are formed and transported with the bloodstream to remote, smaller blood vessels where they can then result in a so-called embolism, that is to say a constriction or blockage.
In order to obviate the problem of clot formation, it is provided in relation to known implants, in particular in relation to the above-mentioned stents, that one or more anti-thrombogenic substances are also to be liberated from a suitable coating in the manner referred to above.
In that respect however there is the problem that on the one hand the anti-thrombogenic substance is comparatively quickly transported away after it has been liberated, by virtue of the constantly occurring flow of blood, and therefore can achieve only a slight effect. The consequence of this is that a relatively large amount of the anti-thrombogenic substance has to be bound in the coating and liberated therefrom in order to achieve an effect which prevents the formation of clots.
On the other hand the constant flow of blood causes the active substance-carrying coating to be also relatively quickly washed out or cleared away so that the required long-term medication with an anti-thrombogenic effect cannot be achieved or can be achieved only at considerable cost, for example by means of a multiple coating with different levels of active substance concentration or delivery rates.
Therefore the object of the present invention is to provide an implant of the general kind set forth, which with simple means permits a long-term reduction in the risk of clot formation.